As furnaces are designed, a special emphasis is laid on their environmental protection features and specifically, on the furnaces being capable of providing combustion regimes that would minimize the amount of furnace compounds vented to the atmosphere.
The nitrogen oxide content of flue gases may be reduced, directly as the fuel is burned, by proper arrangement of the combustion process, i.e. using comparatively simple and economical methods, without the necessity to resort to a complex, bulky and expensive auxiliary equipment. Thus, according to the latest views, a reduced concentration of nitrogen oxides in combustion products may be achieved by an optimum arrangement of three major zones in the flame, namely: ignition and active-combustion zone, reduction zone and oxidation (reburning) zone.
Known in the art (see USSR Author's Certificate No.483559) is a furnace comprising a combustion chamber with a burner for supplying the fuel-air mixture mounted on its sidewall. The slopes of the walls in the lower region of the combustion chamber define a V-type ash hopper with a slot-like mouth. A bottom blast device formed by, say, an air nozzle is disposed beneath the ash hopper mouth.
During operation of such furnace, a fuel-air mixture with a less-than-unity excess-air coefficient is supplied through the burner, while from beneath, through the slot mouth, part of the air necessary for fuel combustion is supplied by the bottom blast device. As a result of interaction between two mutually opposing flows, a turbulent zone is formed throughout the lower region of the furnace, whereas a parallel-flow zone is formed in the upper region thereof. An ignition and active-combustion zone is located adjacent the burner. It is within this combustion zone that the bulk of the fine particles of the fuel is burnt out. The medium-sized and coarse particles of the fuel are separated into the turbulent zone. In the turbulent zone, these particles are burnt out in the course of recycling, After burning down to a certain size, they are removed beyond, the turbulent zone and finally burnt in the upper - parallel-flow - part of the flame. A major portion of the turbulent zone is characterized by a relative deficiency in oxygen and serves as a reduction zone, while the oxygen-rich parallel-flow zone serves as a reburning zone. In other words, a ste-by-step burning of the fuel is conducted in such furnace.
Thus, through the arrangement of the above combustion zones in the furnace by controlling the supply of the fuel of a particular fraction composition and by selecting an appropriate bottom blast rate, a fairly small nitrogen oxide content of flue gases may be ensured.
The above mentioned features of a vortex furnace, however, fail to provide a reduction of the sulphur oxide content of flue gases, since it is virtual impossible to achieve this end using purely aerodynamic and structural techniques.
Special problems arise when burning high-volatile organic fuels. This is due to the fact that as such fuel is heated, an excessive amount of explosive gases is released in the pulverization system, so that a special care is required when selecting the proper scheme of the pulverization process and supplying said fuel to the combustion chamber. Specifically, furnace (inert) gases, rather than air (containing much oxygen), are generally used for drying such fuels.
Known in the art is a furnace for burning solid organic fuel described in the book: R. G. Sach "Boiler Plants" Energy, Moscow, 1968, p.77, comprising a prism-shaped furnace chamber with at least one burner mounted on its wall. The furnace is equipped with a fuel chute serving to supply the fuel to a vertical gas-intake shaft. The gas-intake shaft communicates at the top through a gas-intake window, by a special duct, with the inner space of the furnace chamber. The gas-intake window is generally disposed in the upper region of the furnace chamber. The lower end of the gas-intake shaft communicates with a pulverizing fan for grinding the fuel. The pulverizing fan in turn communicates with a burner for supplying fuel to the furnace chamber.
As the furnace is operated, the flue gases from the top of the combustion chamber are fed through the gas-intake window and the special duct to the gas-intake shaft in which the fuel supplied from the chute is predried and otherwise prepared, by the action of the high temperature of these gases. In this case, there occurs a partial release from the fuel of volatile matter which is mixed with oxygen-deficient inert flue gases. The prepared fuel is transferred to the pulverizer where it is ground to the required fineness and then finally dried. The fuel, along with the gas mixture, is then supplied through the burner to the furnace chamber wherein it is burned together with the volatile matter that had been released before. Since the release of volatile matter from the fuel, as it is passed through the gas-intake shaft, occurs in the flue gas atmosphere having a relatively small concentration of oxygen, no explosive volatile-and-oxygen mixture can be formed under these conditions, thus preventing the risk of explosions and providing the safety of the fuel pulverization system and the furnace itself.
This furnace is rather cost-effective, since effluent gases may be partially used for fuel preparation and drying, and also it may be regarded as environment-friendly, because a complete fuel combustion is ensured at comparatively low temperatures, but provided it is a low-sulphur fuel. Otherwise, this furnace would require additional measures to minimize the sulphur oxide content of the exit gases.
Currently, three basic schemes are largely employed to minimize the amount of sulphur in the flue gas: either sulphur is removed from the fuel prior to feeding it to the furnace (generally, at the spot where it is produced), or various calcium- and magnesium-bearing absorbents, such as lime, calcium carbide etc., are used for cleaning the flue gas beyond the boiler or, finally these absorbents are immediately injected into the furnace chamber for direct (dry or semi-dry) binding the sulphur. Besides, there are composite schemes of binding the sulphur contained in organic fuel. Since calcium compounds belong to low-melting substances, it is essential to feed the absorbent particles to those regions of the furnace in which the temperature does no exceed the absorbent melting point: otherwise, the absorbent particle surface would be fused with the consequent clogging of the pores and reduced reaction area . This might result in a less economical operation of the furnace due to the slagging of water walls and even in a complete shutdown of the boiler.
Known in the art is a furnace realizing a method of simultaneous removal of sulphur and nitrogen from the combustion products. The method is disclosed in the Japanese Patent No. 4-67085.
The furnace comprises a combustion chamber with at least one burner mounted on its wall for supplying the air-fuel mixture. The furnace is provided with an absorbent-feeding means represening a duct for supplying the furnace with finely dispersed or slurry-like calcium-bearing substances for binding sulphur-bearing compounds, which lies above the burner level on the same wall. In addition, the design provides a special equipment for recovery of fly ash from the fuel combustion products, special treatment of this ash and its return to the combustion zone for recycling.
As such furnace is operated, the air-fuel mixture is supplied to the combustion chamber through the burner and the sulphur-absorbing agent is conveyed through an appropriate duct. The absorbent is received by the combustion chamber at a temperature within the range of 900 to 1200.degree. C. In close proximity to the absorbent-feeding duct, a sulphur-binding reaction occurs. The gaseous combustion products then enter the smoke flue with a special device provided therein for the recovery of fly ash, followed by adding acid to part of the ash recovered to neutralize the unreacted calcium oxide or carbonate, and this ash then goes to waste. Ammonium or urea (or its compounds) is added to the remaining part of the ash and the ash is returned to the furnace, to a region with temperatures ranging from 500 to 1000 .degree. disposed at the outlet of the combustion chamber (already beyond its limits). In this region, an additional simultaneous binding of sulphur and, partially nitrogen (out of oxides) occurs.
In this furnace, combustion is carried out within the parallel-flow zone, which accounts for a relatively small residence time of the fuel and absorbent particles within the combustion chamber and hence, a short time of interaction between the absorbent and the flue gases. In these conditions, an effective binding of sulphur is only possible if both the fuel and the absorbent have been subjected to a thorough preparation and its uniform, finely dispersed, structure ensured. Again, the thorough pulverization of the absorbent is also required to obtain its maximum surface area and hence, its full utilization, because the sulphur-binding reaction takes place, largely, on the surface. For such reaction to proceed over the entire surface of an absorbent particle takes more time than the period during which the particle stays within a temperature zone most favourable in terms of the sulphur-binding reaction conditions, namely: 600 . . . 1100.degree. C. Furthermore, in the presence of coarse particles of both the fuel and the absorbent, these particles will not be carried from the furnace entrained in the flue gas but will rather fall through the mouth at the bottom of the combustion chamber and be removed along with the slag, with the consequently impaired economical and environmental-control performance of such furnace. Even a thorough preparation of the absorbing material, however, fails to provide such degree of pulverization that all the absorbent particles react to the full with the sulphur oxides. There always is a certain amount of relatively coarser particles having a layer of reacted absorbent formed on their surface, while the core portion does not take part in the sulphur-binding reaction. This leads to an increased consumption of the expensive absorbent and a lower economical and ecological performance of the furnace. In addition, a complicated system of the post-cleaning of gases after the boiler also results in higher production costs.